Real-world analog signals such as temperature, pressure, sound, or images are routinely converted to a digital representation that can be easily processed in modern digital systems. In many systems, this digital information must be converted back to an analog form to perform some real-world function. The circuits that perform this step are digital-to-analog converters (DACs), and their outputs are used to drive a variety of devices. Loudspeakers, video displays, motors, mechanical servos, radio frequency (RF) transmitters, and temperature controls are just a few diverse examples. DACs are often incorporated into digital systems in which real-world signals are digitized by analog-to-digital converters (ADCs), processed, and then converted back to analog form by DACs. In these systems, the performance of the DACs can be influenced by the capabilities and requirements of the other components in the system. It will be appreciated that in the context of DACs, there are high precision requirements in the presence of stochastic, or random, and deterministic non-idealities and these and other factors are a key motivator for precision tuning and/or calibration in digital to analog converters.
A DAC produces a quantized (discrete step) analog output in response to a binary digital input code and the analog output is commonly a voltage or a current. To generate the output, a reference quantity (usually the aforementioned voltage or current) is divided into binary and/or linear fractions. Then the digital input drives switches that combine an appropriate number of these fractions to produce the output. The number and size of the fractions reflect the number of possible digital input codes, which is a function of converter resolution or the number of bits (N) in the input code. For N bits, there are 2N possible codes. The analog output of the DAC output is the digital fraction represented as the ratio of the digital input code divided by 2N (or 2N−1 depending on the specific definition used) times the analog reference value.
To ensure DAC performance matches expectations, the DAC can be calibrated to ensure that the output analog values correspond with the input digital code. Conventionally this calibration is achieved using full system type calibrations but this can require complex external circuitry.
Overview
A unique DAC architecture comprises calibration components in the form of a Built-In-Self-Test (BIST) and calibration system which can provide an intrinsic or ‘zero’ reference factor. The reference factor can be used to self-reference DAC linearity measurements, thus providing self-calibration. The BIST and calibration system are configured to use actual outputs from the DAC to self-calibrate the DAC. In this way, there is no need for external referencing components to be used in a calibration of the DAC transfer function.
The first and second outputs from the DAC are compared in the analog environment and this comparison is then used as a feedback to a digital signal processor (DSP) component of the DAC architecture. The DSP comprises digital signal processing circuitry which is configured to provide the DAC input codes and compare in a digital environment feedback signals resultant from those codes as part of a calibration routine. A DSP functional block can comprise circuit elements that are configured to provide a mathematical manipulation of a digital signal to modify or improve it. A DSP may also include timing or other control circuit elements, e.g. for testing purposes.
The first and second outputs can be time delimited, i.e., an output at a first time T1 and a second output at a second time T2. In another configuration first and second differential outputs from the DAC taken at the same time can be integrated and their difference checked. In either way, DAC linearity is functionally tested and/or calibrated with only one comparator. By using a digital driven architecture, it is possible to minimize floating-point to fixed-point conversion (FFC) processes and improve precision via digital calibration.